The Palmer Drought Severity Index
(PDSI) and Crop Moisture Index (CMI) are indices of the relative dryness or wetness
effecting water sensitive economies. The PDSI indicates the prolonged and abnormal
moisture deficiency or excess. The CMI gives the short-term or current status of purely
agricultural drought or moisture surplus and can change rapidly from week to week. Both
indicies indicate general conditions and not local variations caused by isolated rain.
Calculation of the PDSI and CMI are made for 350 climatic divisions in the United States
and Puerto Rico. Input to the calculations include the weekly precipitation total and
average temperature, division constants (water capacity of the soil, etc.) and previous
history of the indices.

The PDSI is an important
climatoligical tool for evaluating the scope, severity, and frequency of prolonged periods
of abnormally dry or wet weather. It can be used to help delineate disaster areas and
indicate the availability of irrigation water supplies, reservior levels, range
conditions, amount of stock water, and potential intensity of forest fires. The CMI can be
used to measure the status of dryness or wetness affecting warm season crops and field
activities.

The equation for the index was
empirically derived from the monthly temperature and precipitation scenarios of 13
instances of extreme drought in western Kansas and central Iowa and by assigning an index
value of -4 for these cases. Conversely, a +4 represents extremely wet conditions. From
these values, 7 categories of wet and dry conditions are defined (Table 1). The index is
a sum of the current moisture anomaly and a portion of the previous index to include the
effect of the duration of the drought or wet spell. The moisture anomaly is the product of
a climate weighting factor and the moisture departure. The weighting factor allows the
index to have a reasonably comparable local significance in space and time. A value for a
division in Florida would have the same local implication as a similiar value in a more
arid division in western Kansas. The moisture departure is the difference of water supply
and demand. Supply is precipitation and stored soil moisture and demand is the potential
evapotranspiration, the amount needed to recharge the soil, and runoff needed to keep the
rivers, lakes, and resevoirs at a normal level.

Table 1

-4.0 or less (Extreme Drought)

+2.0 or +2.9 (Unusual Moist Spell)

-3.0 or -3.9 (Severe Drought)

+3.0 or +3.9 (Very Moist Spell)

-2.0 or -2.9 (Moderate Drought)

+4.0 or above (Extremely Moist)

-1.9 to +1.9 (Near Normal)

The duration of the drought (or wet
spell) is determined by calculating indices for different weather spells (incipient and
established wet and dry spells). A week of normal or better rainfall is welcome in an area
that has experienced a long drought but may be only a brief respite and not the end of the
drought. Once a weather spell is established (by computing a 100% "probability"
that an opposite weather spell has ended), the final value is assigned. In order for the
program to have a real-time significance, a value is assigned based on a greater than 50%
"probability" that the opposite weather spell has ended. This is not entirely
satisfactoty, but it does allow the index to have a value when there is a doubt as to
whether it should be positive or negative. A "F" is placed after the PDSI when a
weather spell is established and a "P" when a weather spell is not established.

The CMI was developed from some of
the moisture accounting procedures used in computing the PDSI. This index is the sum of
the evapotranspiration anomaly (which is generally negative or slightly positive) and the
moisture excess (either zero or positive). Both terms are a function of the previous week
and a measure of the current week. The evapotranspiration anomaly is weighted to make it
comparable in space and time. If the potential moisture demand exceeds available moisture
supplies, the CMI is negative. However, if moisture meets or exceeds demand the index is
positive. It is necessary to use two separate legends because the resulting effects are
different when the moisture supply is improving than when it is deteriorating (Table 2).
The stage of crop development and soil type should be considered when using this index. In
irrigated regions, only departures from ordinary irrigation requirements are reflected.

Table 2

-3.0 or less (Severely Dry)

+1.0 or +1.9 (Abnormally Moist)

-2.0 or -2.9 (Excessively Dry)

+2.0 or +2.9 (Wet)

-1.0 or -1.9 (Abnormally Dry)

+3.0 and above (Excessively Wet)

-0.9 or +0.9 (Slightly Dry/Favorably Moist)

A parameter obtained from the
calculations is the monthly moisture anomaly (Z) index which is the product of the
moisture departure of the most recent 4 weeks and a climate weighting factor. This index
can be used as an indicator of forest fire ignition.

Another parameter derived from the
calculations is the additional precipitation in inches needed to bring the PDSI to near
zero. This parameter is computed for all values of the current week's PDSI less than -.5
and left blank for all values greater than or equal to -.5. The precipitation values are
the theoretical, additional amounts required to end the drought in each climatic division.
In using this parameter to make projections, it must be realized that these values are
instantaneous, valid only for the current week. To end the drought in a given climatic
division for the oncoming period, the amount listed plus near-normal rainfall must occur.

The following is a listing of the
parameters in the files and their meaning. Temperature and precipitation are data received
from the field and the other parameter are results of the Palmer drought and crop moisture
data calculations. The week number in the heading is the week of the growing season where
week one is the week with the first Wednesday in March. The computations are reinitated
each year for week one using the output of the February Palmer data run. All initial data
are replaced with the historical data received from the National Climatic Data Center in
Asheville when available and the calculations rerun.